[RFC PATCH] iio: proximity: sx9360: fix hardware gain encoding

[RFC PATCH] iio: proximity: sx9360: fix hardware gain encoding

[Arnav Kapoor]

The sx9360 PROX_CTRL0 gain handling was copied from the original sx9324
driver and has the same bugs that commit 108e4d4de2b5
("iio:proximity:sx9324: Fix hardware gain read/write") fixed there:

  - The gain field holds ilog2(<gain>) + 1, with 0 reserved, so a gain
    of 1 is 0x1 and a gain of 8 is 0x4. read_gain() and write_gain()
    use ilog2(<gain>) instead, so every gain is off by one: writing 1
    programs the reserved code 0 and writing 8 programs a gain of 4.
  - SX9360_REG_PROX_CTRL0_GAIN_1 is 0x80, which sets the reserved bit 7
    and leaves the gain field at the reserved code 0, instead of
    0x1 << 3.

Use the same encoding as sx9324: add one when writing, subtract one when
reading, return -EINVAL for the reserved and out-of-range codes, and
build the default with FIELD_PREP_CONST(). Use BIT() for the read value,
as Andy suggested for the same code in 2022.

Fixes: 1cdb4c47f7f5 ("iio:proximity:sx9360: Add sx9360 support")
Signed-off-by: Arnav Kapoor <kapoorarnav43@gmail.com>
---
RFC because I don't have the SX9360 datasheet. The only public copy I
found (the one linked at the top of sx9360.c) is a short excerpt of the
SX9324 datasheet without the register map, so this follows the sx9324
fix by analogy: the sx9360 gain code is a copy of the pre-fix sx9324
code, with the same mask, the same 0x80 default and the same missing +1.
Gwendal, Stephen: could you confirm that SX9360 PROX_CTRL0 uses the same
gain encoding as SX9324 (bits 5:3, ilog2(gain) + 1, 0 reserved, bit 7
reserved)?

Xiaohui Zhang posted the same change in June 2022 ("iio:proximity:sx9360:
Fix hardware gain read/write"); it had build errors and a dead-code
read path and was not respun. The read path here uses the corrected
check from the sx9324 read_gain() fix I sent on 2026-11-04.

Applies on top of my series "iio: proximity: sx9310/sx9360: reject
unsupported hardware gain values":
https://lore.kernel.org/all/20261107111710.442318-1-kapoorarnav43@gmail.com/

Compile-tested only (W=1); I don't have SX9360 hardware.

 drivers/iio/proximity/sx9360.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/drivers/iio/proximity/sx9360.c b/drivers/iio/proximity/sx9360.c
index c6e5b3e557bb..01a4a94219ec 100644
--- a/drivers/iio/proximity/sx9360.c
+++ b/drivers/iio/proximity/sx9360.c
@@ -67,7 +67,9 @@
 #define SX9360_REG_PROX_CTRL0_PHR	0x40
 #define SX9360_REG_PROX_CTRL0_PHM	0x41
 #define SX9360_REG_PROX_CTRL0_GAIN_MASK	GENMASK(5, 3)
-#define SX9360_REG_PROX_CTRL0_GAIN_1		0x80
+#define SX9360_REG_PROX_CTRL0_GAIN_RSVD		0x0
+#define SX9360_REG_PROX_CTRL0_GAIN_1		0x1
+#define SX9360_REG_PROX_CTRL0_GAIN_8		0x4
 #define SX9360_REG_PROX_CTRL0_RAWFILT_MASK	GENMASK(2, 0)
 #define SX9360_REG_PROX_CTRL0_RAWFILT_1P50	0x01
 #define SX9360_REG_PROX_CTRL1		0x42
@@ -291,7 +293,12 @@ static int sx9360_read_gain(struct sx_common_data *data,
 	if (ret)
 		return ret;
 
-	*val = 1 << FIELD_GET(SX9360_REG_PROX_CTRL0_GAIN_MASK, regval);
+	regval = FIELD_GET(SX9360_REG_PROX_CTRL0_GAIN_MASK, regval);
+	if (regval == SX9360_REG_PROX_CTRL0_GAIN_RSVD ||
+	    regval > SX9360_REG_PROX_CTRL0_GAIN_8)
+		return -EINVAL;
+
+	*val = BIT(regval - 1);
 
 	return IIO_VAL_INT;
 }
@@ -628,7 +635,7 @@ static int sx9360_write_gain(struct sx_common_data *data,
 	if (val <= 0 || val > 8 || !is_power_of_2(val))
 		return -EINVAL;
 
-	gain = ilog2(val);
+	gain = ilog2(val) + 1;
 	reg = SX9360_REG_PROX_CTRL0_PHR + chan->channel;
 	gain = FIELD_PREP(SX9360_REG_PROX_CTRL0_GAIN_MASK, gain);
 
@@ -676,9 +683,11 @@ static const struct sx_common_reg_default sx9360_default_regs[] = {
 	{ SX9360_REG_AFE_PARAM1_PHM, SX9360_REG_AFE_PARAM1_AGAIN_PHM_6PF |
 		SX9360_REG_AFE_PARAM1_FREQ_83_33HZ, "afe_param1_phm" },
 
-	{ SX9360_REG_PROX_CTRL0_PHR, SX9360_REG_PROX_CTRL0_GAIN_1 |
+	{ SX9360_REG_PROX_CTRL0_PHR,
+		FIELD_PREP_CONST(SX9360_REG_PROX_CTRL0_GAIN_MASK, SX9360_REG_PROX_CTRL0_GAIN_1) |
 		SX9360_REG_PROX_CTRL0_RAWFILT_1P50, "prox_ctrl0_phr" },
-	{ SX9360_REG_PROX_CTRL0_PHM, SX9360_REG_PROX_CTRL0_GAIN_1 |
+	{ SX9360_REG_PROX_CTRL0_PHM,
+		FIELD_PREP_CONST(SX9360_REG_PROX_CTRL0_GAIN_MASK, SX9360_REG_PROX_CTRL0_GAIN_1) |
 		SX9360_REG_PROX_CTRL0_RAWFILT_1P50, "prox_ctrl0_phm" },
 	{ SX9360_REG_PROX_CTRL1, SX9360_REG_PROX_CTRL1_AVGNEG_THRESH_16K, "prox_ctrl1" },
 	{ SX9360_REG_PROX_CTRL2, SX9360_REG_PROX_CTRL2_AVGDEB_2SAMPLES |

base-commit: 509224d37ff169a02f388ab6e525d5517fb570dc
-- 
2.53.0